feat(tags): system tags — is_system column, seeded hygiene tags, protection guards

Training hygiene step 1 (milestone #128). Migration 0075 adds
tag.is_system and seeds wip / banner / editor screenshot (kind=general),
ADOPTING an existing same-(name,kind) tag case-insensitively instead of
duplicating. These rows drive the upcoming training exclusions, so they
are protected: rename and merge-away refuse system tags (merge-INTO
stays allowed — folding an operator's old hygiene tag into the system
row is the intended move; merge is the only tag-delete path, so that
guard covers deletion). is_system rides every tag serialization.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/api/tags.py

@@ -324,6 +324,7 @@ async def get_tag(tag_id: int):
                 "name": tag.name,
                 "kind": tag.kind.value,
                 "fandom_id": tag.fandom_id,
+                "is_system": tag.is_system,
             }
         )
 
@@ -390,6 +391,7 @@ async def update_tag(tag_id: int):
                 "name": tag.name,
                 "kind": tag.kind.value,
                 "fandom_id": tag.fandom_id,
+                "is_system": tag.is_system,
             }
         )